The present application claims priority to Japanese Application No. P11-054459 filed Mar. 2, 1999 which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a secondary battery using a nonaqueous electrolyte solution (hereinafter referred to as a xe2x80x9cnonaqueous secondary batteryxe2x80x9d), a method for making a negative electrode component used in the nonaqueous secondary battery, an apparatus for evaluating a graphite material for the negative electrode component, and an apparatus for making the graphite material.
2. Description of the Related Art
As rapid progress is made in the miniaturization of electronic devices, such as portable phones and notebook personal computers, secondary batteries are required to have higher energy densities.
In conventional secondary batteries, such as lead batteries, Nixe2x80x94Cd batteries, and Nixe2x80x94MH batteries, discharge voltages are low and energy densities are insufficient. Lithium secondary batteries are also used in practice, in which metallic lithium, lithium alloys, and carbonaceous materials which can electrochemically occlude and release lithium ions are used as negative electrode active materials, and various positive electrodes are used. The lithium secondary batteries have high output voltages and thus have large energy densities per weight or volume compared to the above conventional batteries.
In the lithium secondary batteries at initial stages, metallic lithium and lithium alloys are used as negative electrodes. A negative electrode using metallic lithium or a lithium alloy is insufficient in charge-discharge efficiency and has a problem in that dendritic lithium is formed. Thus, such negative electrodes are used in practice only in a few specialized fields.
Carbonaceous materials which can electrochemically occlude and release lithium ions have recently been anticipated as negative electrode components and are now coming into use. Negative electrodes using the carbonaceous materials do not have problems inherent in the metallic lithium or lithium alloys, that is, the formation of metallic lithium having a dendritic structure and particularization of the lithium alloy during charge-discharge cycles. Moreover, the carbonaceous materials show high coulomb efficiency; hence, lithium secondary batteries having carbonaceous negative electrodes exhibit superior charge-discharge reversibility.
In secondary batteries using the carbonaceous materials as negative electrode active materials, metallic lithium is not precipitated in use. Thus, lithium secondary batteries using the carbonaceous materials and nonflammable lithium compound oxide are safe and are commercially produced. These batteries are called xe2x80x9clithium ion batteriesxe2x80x9d and use carbonaceous materials as negative electrodes, LiCoO2 as positive electrodes, and nonaqueous electrolyte solutions containing nonaqueous solvents.
Carbonaceous materials used as negative electrodes are classified into graphite materials including natural products and artificial products, easily-graphitizable carbonaceous materials as precursors of artificial graphite materials, and ungraphitizable carbonaceous materials which are not converted to graphite even when these are treated at high temperatures facilitating the formation of graphite. Graphite materials and ungraphitizable carbonaceous materials have high capacities as negative electrodes and are thus currently used.
Lithium ion batteries have rapidly gained widespread use as electrical power sources in electronic devices, particularly, notebook personal computers due to compact, because they are lightweight, and have high capacities. Notebook personal computers having improved performance require higher CPU clock frequencies. Thus, high-performance computers consume significant amounts of electrical power and generate substantial amounts of heat during operation. Moreover, the restricted volume of dead space, which is inherent in miniaturization of personal computers, precludes the dissipation of heat which is generated during operation, resulting in an increase in the internal temperatures of personal computers.
The increased temperature accelerates deterioration and thus decreases capacity in batteries used in personal computers. The lost capacity cannot be restored by any means.
Accordingly, it is an object of the present invention to provide a lithium ion secondary battery which does not cause deterioration by generating excessive heat, has high capacity, and is highly reliable in use.
It is another object of the present invention to provide a method for making a negative electrode component used in a nonaqueous secondary battery, an apparatus for evaluating a graphite material as the negative electrode component, and an apparatus for making the graphite material.
According to experimental results obtained by the present inventors, a lithium ion secondary battery using a carbonaceous material for a negative electrode, which has specific structural parameters, can maintain high capacity even after such a battery is stored in high-temperature environments, and has high capacity reliability for long periods.
According to a first aspect of the present invention, a nonaqueous secondary battery includes a negative electrode including a carbonaceous material in which the ratio RG=Gs/Gb of the degree of graphitization Gs of the carbonaceous material, determined by a surface-enhanced Raman spectrum, to the degree of graphitization Gb, determined by a Raman spectrum measured using argon laser light, is at least 4.5, based on the following conditions:
Gb=Hbb/Hba;
and
Gs=Hsb/Hsa;
wherein
Hba is the height of a peak lying in a range of 1,580 cmxe2x88x921 to 1,620 cmxe2x88x921 in a Raman spectrum which is measured using an argon laser Raman spectrometer of a wavelength of 514.5 nm and a wavelength resolution of 4 cmxe2x88x921;
Hbb is the height of a peak lying in a range of 1,350 cmxe2x88x921 to 1,400 cmxe2x88x921 in a Raman spectrum which is measured using an argon laser Raman spectrometer of a wavelength of 514.5 nm and a wavelength resolution of 4 cmxe2x88x921;
Hsa is the height of a peak lying in a range of 1,580 cmxe2x88x921 to 1,620 cmxe2x88x921 in a surface-enhanced Raman spectrum which is measured using an argon laser Raman spectrometer of a wavelength of 514.5 nm and a wavelength resolution of 4 cmxe2x88x921 when silver having a thickness of 10 nm is deposited on the carbonaceous material; and
Hsb is the height of a peak lying in a range of 1,350 cmxe2x88x921 to 1,400 cmxe2x88x921 in a surface-enhanced Raman spectrum which is measured using an argon laser Raman spectrometer of a wavelength of 514.5 nm and a wavelength resolution of 4 cmxe2x88x921 when silver having a thickness of 10 nm is deposited on the carbonaceous material.
According to a second aspect of the present invention, a nonaqueous secondary battery includes a negative electrode including a carbonaceous material having a peak in a wavelength range above 1,360 cmxe2x88x921 in a surface-enhanced Raman spectrum which is measured using an argon laser Raman spectrometer of a wavelength of 514.5 nm and a wavelength resolution of 4 cmxe2x88x921 when silver having a thickness of 10 nm is deposited on the carbonaceous material.
In the nonaqueous secondary battery according to the first aspect and second aspect, the carbonaceous material is preferably graphite.
The nonaqueous secondary battery further includes a positive electrode, which preferably includes a lithium compound oxide represented by LiMxOy wherein M is at least one element selected from the group consisting of Co, Ni, Mn, Fe, Cr, Al, and Ti.
According to a third aspect of the present invention, a method for making a negative electrode component used in the nonaqueous secondary battery according to the first aspect, includes the steps of carbonizing a raw material, slightly oxidizing the surface of the carbonized material, and then graphitizing the oxidized material.
According to a fourth aspect of the present invention, a method for making a negative electrode component used in the nonaqueous secondary battery according to the second aspect, includes the steps of carbonizing a raw material, slightly oxidizing the surface of the carbonized material, and then graphitizing the oxidized material.
According to a fifth aspect of the present invention, a method for making a negative electrode component used in the nonaqueous secondary battery according to the first aspect, includes the step of polishing the surface of the negative electrode component by irradiating the surface with light.
According to a sixth aspect of the present invention, a method for making a negative electrode component used in the nonaqueous secondary battery according to the second aspect, includes the step of polishing the surface of the negative electrode component by irradiating the surface with light.
According to a seventh aspect of the present invention, a method for making a negative electrode component used in the nonaqueous secondary battery according to the first aspect, includes the step of annealing natural graphite having a rhombic structure at a temperature of at least 2,000xc2x0 C.
According to an eighth aspect of the present invention, a method for making a negative electrode component used in the nonaqueous secondary battery according to the second aspect, includes the step of annealing natural graphite having a rhombic structure at a temperature of at least 2,000xc2x0 C.
According to a ninth aspect of the present invention, an apparatus for evaluating a graphite material includes a surface-enhanced Raman spectroscopic means.
According to a tenth aspect of the present invention, an apparatus for evaluating a graphite material used in a nonaqueous secondary battery, determines whether the ratio RG=Gs/Gb of the degree of graphitization Gs of the carbonaceous material, determined by a surface-enhanced Raman spectrum, to the degree of graphitization Gb, determined by a Raman spectrum measured using argon laser light, is at least 4.5, based on the conditions described in the first aspect.
According to an eleventh aspect of the present invention, an apparatus for evaluating a graphite material used in a nonaqueous secondary battery, determines if the graphite material has a peak in a wavelength range above 1,360 cmxe2x88x921 in a surface-enhanced Raman spectrum which is measured using an argon laser Raman spectrometer of a wavelength of 514.5 nm and a wavelength resolution of 4 cmxe2x88x921 when silver having a thickness of 10 nm is deposited on the carbonaceous material.
According to a twelfth aspect of the present invention, an apparatus for making a graphite material includes an apparatus for evaluating a graphite material according to the ninth aspect.
According to a thirteenth aspect of the present invention, an apparatus for making a graphite material includes an apparatus for evaluating a graphite material according to the tenth aspect.
According to a fourteenth aspect of the present invention, an apparatus for making a graphite material includes an apparatus for evaluating a graphite material according to the eleventh aspect.
The method for measuring the structural parameters characterized by the present invention is described below. The structural parameters are determined by Raman spectroscopy. The Raman spectrum of a conventional carbonaceous material has a peak Pba in a range of 1,580 cmxe2x88x921 to 1,620 cmxe2x88x921 assigned to a graphite crystal structure and a peak Pbb in a range of 1,358 cmxe2x88x921 to 1,400 cmxe2x88x921 assigned to an amorphous structure. When the graphite structure is disordered, the intensity Hba of the peak Pba decreases, whereas the intensity Hbb of the peak Pbb increases. Thus, the ratio of the height Hba to the height Hbb represents the degree of graphitization.
Surface-enhanced Raman spectrometry (SERS) was developed by Fleischmann et al. in 1974. In this method, a thin metal layer is deposited on the surface of a sample to be measured. This method is characterized by uppermost surface analysis on the order of several nanometers and enhanced Raman sensitivity. A surface-enhanced Raman spectrum is substantially the same as the corresponding conventional Raman spectrum, although the analytical depth differs between these methods. That is, the surface-enhanced Raman spectrum has a peak Pba in a range of 1,580 cmxe2x88x921 to 1,620 cmxe2x88x921 assigned to a graphite crystal structure and a peak Pbb in a range of 1,350 cmxe2x88x921 to 1,400 cmxe2x88x921 assigned to an amorphous structure. The ratio of the intensity (height Hba) of the peak Psa to the intensity (height Hbb) of the peak Psb represents the degree of graphitization at the topmost layer.
The carbonaceous material for the negative electrode component in the nonaqueous secondary battery in accordance with the present invention is characterized by the ratio of the intensities measured by the two types of Raman spectroscopy, and the capacity of the battery can be maintained even after the battery is stored in high-temperature environments.
Such a carbonaceous material can be produced by the method for making the negative electrode component in accordance with the present invention.
The apparatus for evaluating the graphite material including a surface-enhanced Raman spectrometer can evaluate the graphite material before a battery is fabricated using the graphite material.
The apparatus for making the graphite material including the apparatus for evaluating the graphite material contributes to improved quality control in the manufacturing process, optimization of the process, and increasing the rate of development of the graphite material.